Large volumes of water are necessary and are used for froth flotation in mining operations. In many areas water is scarce and becoming more so, and this is particularly true in the Southwestern United States; especially in Arizona where there are large copper mining facilities.
In an attempt to supplement the limited water supply, efforts have been made to use the water effluent from sewage treatment plants in ore flotation operations. These efforts have not been successful since it has been found that, while on some occasions the effluent is suitable, on the vast majority of occasions such effluent is extremely detrimental to the process of ore flotation in which the majority of water is used in ore processing.
The precise reasons for this detrimental effect are not known, but it is speculated that detergents, surfactants, fatty acids, and/or other organic hydrocarbons present in the sewage effluent affect the natural hydrophilic qualities of the gangue material present with the ore so that the gangue is activated and floats in the ore concentrate; the gangue is not depressed as in the usual flotation operations.
Treatments of the sewage effluent have been tried to reduce this detrimental effect, but they have not been successful or desirable for commercial operations mainly because of cost or because the particular treatment is not capable of coping with the wide variation in materials and in concentrations of materials present in sewage effluent from day to day.
Thus, for example, hydrated lime has been added to the effluent to precipitate undesirable materials, but it has been found that large amounts of lime are required to minimize the adverse effects of the sewage effluent on flotation and, also, additional equipment in the form of settlers to remove the lime precipitate from the effluent before use. The result is a cost of more than 25 cents per 1,000 gallons of effluent treated.
Another procedure tried has been physical treatment of the sewage effluent prior to use. The effluent has been processed by a thickener type operation followed by aeration and additional settling. The clarified effluent was then used for flotation, but still found to have a detrimental effect. It has been found that a minimum of four days of aeration were needed to completely reduce the adverse effects of the effluent. The cost in providing sufficient aeration reservoirs and settling equipment is such that this treatment procedure is not commercially desirable. To use sewage effluent at 2,000 gpm, four separate aeration reservoirs with a capacity of 3 million gallons each would be required.